Ive done some more reading on this and found out this info, the TX4D is the FAT MOD or Searing Boogie Lead by the looks of it.

Here is what i found:

Mesa changed the mode adding a new board (TX-4) that emulated the Rectifier sound and named the LD1 Red "Recto Vintage", and later on they modified the TX4 board to sound warmer and more balanced like the Recto ORGN Mode in a Rectifier... They called that modification "The Fat Mode". This TX-4 board had 4 different revisions (A, B, C, D). Presumably all sound the same... People claims that the "D" is the better sounding (With or without the Fat Mod).

In Q3 of 2004 Mesa run out of supplies to make the TX-4 board so they replaced it with th TX-5 Board and named the LD1 Red "Searing Boogie Lead". That mode is based on the Lonestar. That's the least popular of them all.

If I recall correctly, the Triaxis modelled in the Axe has the TX-4 board and the Fat Mod.